Ultrasonic flow meters are used to measure the flow velocity of a medium, such as gas, flowing through a conduit. A transit-time or time-of-flight ultrasonic flow meter uses the time of travel for both an ultrasonic upstream signal (defined to be substantially against the flow of the medium) and downstream ultrasonic signal (in the opposite direction as upstream) between the two transceivers to determine the flow velocity of the medium in the conduit.
Downstream signals normally produce better signal-to-noise ratio (SNR) than upstream signals, especially at high flow velocities. That is, the upstream ultrasonic signal has lower signal-to-noise ratio (SNR) than the downstream ultrasonic signal, especially as the velocity of the flow of the medium increases. A downstream only flow meter uses downstream signals transmitted by two ultrasonic emitters to two receivers to measure the flow velocity of the medium.